Airlines just haven't been eager to jump through the hoops and red tape — or pony up the money — for the testing that would let their customers play Mumford & Sons all flight long.

"They have been able to go through the process for allowing [gadget use],” said one FAA source. The airlines have simply chosen not to, in large part because of the cost.

Still questions remain about how to let passengers use their favorite devices while protecting safety, a goal everyone involved in the issue supports. The basic concern is that a signal from an electronic device could interfere with avionics or communications equipment on an airplane, causing it to malfunction.

Cruising at 30,000 feet, a pilot could easily handle a glitch on a cockpit screen that momentarily appears to shift the plane’s pitch, for instance. But that scenario is particularly dangerous during the riskiest phases of flight — takeoff and landing.

Regulators and researchers also insist that while one errant signal — such as from a pilot’s iPad — probably wouldn’t be enough to down a plane, 150 of them bombarding the plane’s equipment is another matter entirely.

“Two iPads, they can test that and make sure those are kosher,” said one Senate aide. “But 300 iPads, we’re not sure what that means — or a mix of devices for an additive effect, that could be a problem.”

Critics of the current approach also say the problem is with the FAA's engineer mindset, which requires an airline to prove the safety of a dizzying array of combinations — say, an iPhone 5 on a Boeing 787, or a Kindle Fire HD on an Airbus A380 — all of which add up to an expensive proposition.

Even if an airline went through the expense of testing and data-gathering, there's no guarantee the FAA would find the results adequate.